Abstract:

A phase-changeable memory device includes a phase-changeable material
pattern and first and second electrodes electrically connected to the
phase-changeable material pattern. The first and second electrodes are
configured to provide an electrical signal to the phase-changeable
material pattern. The phase-changeable material pattern includes a first
phase-changeable material layer and a second phase-changeable material
layer. The first and second phase-changeable material patterns have
different chemical, physical, and/or electrical characteristics. For
example, the second phase-changeable material layer may have a greater
resistivity than the first phase-changeable material layer. For instance,
the first phase-changeable material layer may include nitrogen at a first
concentration, and the second phase-changeable material layer may include
nitrogen at a second concentration that is greater than the first
concentration. Related devices and fabrication methods are also
discussed.

Claims:

1. A phase-changeable memory device, comprising:a first phase-changeable
layer on a first electrode; anda second phase-changeable layer on the
first phase-changeable layer, a grain size of the second phase-changeable
layer being smaller than a grain size of the first phase-changeable
layer.

2. The device of claim 1, further comprising:the first electrode and a
second electrode with interposing the first and the second
phase-changeable layers therebetween to provide an electrical signal to
the first and second phase-changeable layers; andan adhesive layer
between the second electrode and the second phase-changeable layer.

4. The device of claim 3, wherein the second phase-changeable layer
includes an atom of the adhesive layer.

5. The device of claim 3, wherein the second phase-changeable layer
includes a higher atomic percentage of titanium, zirconium, tungsten,
molybdenum, tantalum, copper, and/or aluminum than the first
phase-changeable layer for the same atom.

6. The device of claim 2, wherein a phase-change is induced in the first
phase-changeable layer when a predetermined current is applied to the
first and second phase-changeable layers via the first and second
electrodes.

7. The device of claim 6, wherein the second phase-changeable layer
includes a higher atomic percentage of titanium, zirconium, tungsten,
molybdenum, tantalum, copper, and/or aluminum than the first
phase-changeable layer for the same atom.

8. The device of claim 7, wherein a phase-change is not induced in the
second phase-changeable layer when a predetermined current is applied to
the first and second phase-changeable layers via the first and second
electrodes.

9. The device of claim 6, wherein a phase-change is not induced in the
second phase-changeable layer when a predetermined current is applied to
the first and second phase-changeable layers via the first and second
electrodes.

10. The device of claim 1, wherein a phase-change is induced in the first
phase-changeable layer when a predetermined current is applied to the
first and second phase-changeable layers via the first and second
electrodes.

11. The device of claim 10, wherein the second phase-changeable layer
includes a higher atomic percentage of titanium, zirconium, tungsten,
molybdenum, tantalum, copper, and/or aluminum than the first
phase-changeable layer for the same atom.

12. The device of claim 11, wherein a phase-change is not induced in the
second phase-changeable layer when a predetermined current is applied to
the first and second phase-changeable layers via the first and second
electrodes.

13. The device of claim 10, wherein a phase-change is not induced in the
second phase-changeable layer when a predetermined current is applied to
the first and second phase-changeable layers via the first and second
electrodes.

14. The device of claim 2, wherein first phase-changeable layer is
directly on the second phase-changeable layer opposite the adhesive
layer, and wherein the second phase-changeable layer is configured to
substantially prevent atoms of the adhesive layer from diffusing into the
first phase-changeable layer.

15. The device of claim 2, wherein the first and/or second
phase-changeable layers comprise a chalcogenide compound.

16. The device of claim 15, wherein the second phase-changeable layer
includes a higher atomic percentage of an atom of the adhesive layer than
the first phase-changeable layer for the same atom.

17. The device of claim 16, wherein a phase-change is induced in the first
phase-changeable layer but not in the second phase-changeable layer when
a predetermined current is applied to the first and second
phase-changeable layers via the first and second electrodes.

18. The device of claim 15, wherein a phase-change is induced in the first
phase-changeable layer but not in the second phase-changeable layer when
a predetermined current is applied to the first and second
phase-changeable layers via the first and second electrodes.

19. A phase-changeable memory device, comprising:a first electrode;a first
phase-changeable layer on the first electrode;a second phase-changeable
layer on the first phase-changeable layer, the second phase-changeable
layer including an impurity at a higher concentration than the first
phase-changeable layer; anda second electrode on the second
phase-changeable layer.

20. A phase-changeable memory device, comprising:a first electrode;a first
phase-changeable layer on the first electrode;a second phase-changeable
layer on the first phase-changeable layer; anda second electrode on the
second phase-changeable layer,wherein the first phase-changeable layer
and the second phase-changeable layer include an impurity at a different
concentration such that a phase-change is induced in the first
phase-changeable layer but not in the second phase-changeable layer when
a predetermined current is applied to the first and second
phase-changeable layers via the first and second electrodes.

Description:

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001]This application is a continuation of and claims priority from U.S.
patent application Ser. No. 12/189,477, filed Aug. 11, 2008. U.S. patent
application Ser. No. 12/189,477 is a divisional of and claims priority
from U.S. patent application Ser. No. 11/627,775, filed Jan. 26, 2007.
U.S. patent application Ser. No. 11/627,775 is a continuation-in-part of
and claims priority from U.S. patent application Ser. No. 10/910,945,
filed Aug. 4, 2004, which claims priority from Korean Patent Application
No. 2004-12358, filed on Feb. 24, 2004. U.S. patent application Ser. No.
10/910,945 is a continuation-in-part of and claims priority from U.S.
patent application Ser. No. 10/781,597, filed Feb. 18, 2004, which claims
priority from Korean Patent Application No. 2003-11416, filed on Feb. 24,
2003. The present application also claims priority under 35 U.S.C.
§119 from Korean Patent Application No. 2006-0008674, filed on Jan.
27, 2006 in Korean Intellectual Property Office. The present application
thus claims the benefit of priority from all of the above-referenced U.S.
and Korean applications, and the disclosures of all of the
above-referenced U.S. and Korean applications are hereby incorporated by
reference herein in their entireties.

FIELD OF THE INVENTION

[0002]The present invention relates to non-volatile memory devices, and
more particularly, to phase-changeable memory devices and methods for
fabricating the same.

BACKGROUND OF THE INVENTION

[0003]A phase-changeable memory device is a kind of non-volatile memory
device. Phase-changeable memory devices may use a phase-changeable
material such as a chalcogenide compound that can be switched between a
crystalline state and an amorphous state based on applied electrical
signals, thereby exhibiting a high-resistance state and a low-resistance
state that can be distinguished from each other. In response to applied
voltages, current flow through the phase-changeable material may heat the
phase-changeable material. Depending on the heating temperature and
duration, the phase-changeable material may be changed to a program state
of a high-resistance (i.e., a reset state) or a low-resistance (i.e., a
set state). The high-resistance state and the low-resistance state may be
switched and/or reversed based on the applied current.

[0004]The heating temperature of the phase-changeable material may be
proportional to the amount of applied current. In order to achieve a
relatively high density of integration, it may be desirable to reduce the
temperature by reducing a program current flow. More particularly, it may
be desirable to reduce the switching current for the reset state, as
switching to the reset state may require more current than switching to
the set state.

SUMMARY OF THE INVENTION

[0005]According to some embodiments of the present invention, a
phase-changeable memory device includes a phase-changeable material
pattern, and first and second electrodes electrically connected to the
phase-changeable material pattern that provide an electrical signal to
the phase-changeable material pattern. The phase-changeable material
pattern includes a first phase-changeable material layer and a second
phase-changeable material layer on the first phase changeable material
layer. The first and second phase-changeable material layers have
different chemical, physical, and/or electrical characteristics. For
example, the second phase-changeable material layer may have a greater
resistivity than the first phase-changeable material layer.

[0006]According to other embodiments of the present invention, a
phase-changeable memory device includes a phase-changeable material
pattern, and first and second electrodes electrically connected to the
phase-changeable material pattern. The first and second electrodes are
configured to provide an electrical signal to the phase-changeable
material pattern. The phase-changeable material pattern includes a first
phase-changeable material layer including nitrogen at a first
concentration, and a second phase-changeable material layer including
nitrogen at a second concentration that is greater than the first
concentration.

[0007]In some embodiments, the first concentration of nitrogen may be from
about 0 to about 5 atomic percent of the first phase-changeable material
layer. In addition, the second concentration of nitrogen may be from
about 5 to about 20 atomic percent of the second phase-changeable
material layer.

[0008]In other embodiments, the first and second concentrations of
nitrogen may be selected such that a phase change is induced in the first
phase-changeable material layer but not in the second phase-changeable
material layer when a predetermined current is applied to the
phase-changeable material pattern via the first and second electrodes.

[0009]In some embodiments, the second phase-changeable material layer may
further include a conductive material. For example, the second electrode
may be on the second phase-changeable material layer, and an adhesive
layer including the conductive material may be between the second
electrode and the second phase-changeable material layer. The first
phase-changeable material layer may be directly on the second
phase-changeable material layer opposite the adhesive layer, and the
second phase-changeable material layer may be configured to substantially
prevent portions of the adhesive layer from diffusing into the first
phase-changeable material layer. As such, the second phase-changeable
material layer may include a higher atomic percentage of the conductive
material than the first phase-changeable material layer.

[0010]In other embodiments, a grain size of the second phase-changeable
material layer may be smaller than a grain size of the first
phase-changeable material layer.

[0011]In some embodiments, the second phase-changeable material layer may
have a lower thermal conductivity than the first phase-changeable
material layer.

[0012]In other embodiments, the second phase-changeable material layer may
be (GeaSbbTe100-(a+b))nN100-n, where
80≦n≦95, where a, b and 100-(a+b) may be atomic percentages
with respect to the Ge--Sb--Te composition, and where n and 100-n may be
atomic percentages with respect to the total composition of the second
phase-changeable material layer.

[0013]According to further embodiments of the present invention, a method
of forming a phase-changeable memory device includes forming a first
electrode, forming a first phase-changeable material layer on the first
electrode, forming a second phase-changeable material layer on the first
phase-changeable material layer to define a phase-changeable material
pattern, and forming a second electrode on the second phase-changeable
material layer such that the first and second electrodes are electrically
connected to the phase-changeable material pattern and are configured to
provide an electrical signal to the phase-changeable material pattern.
The first phase-changeable layer includes nitrogen at a first
concentration, and the second phase-changeable layer includes nitrogen at
a second concentration, where the second concentration of nitrogen is
greater than the first concentration.

[0014]In some embodiments, forming the first and/or second
phase-changeable material layers may include sputtering a
phase-changeable material onto a substrate using an argon sputtering gas
and a nitrogen source gas to form the first and/or second
phase-changeable material layers on the substrate. For example, the
nitrogen source gas may be supplied to the substrate at a first flow rate
to deposit the first phase-changeable material layer thereon. In
addition, the nitrogen source gas may be supplied to the substrate at a
second flow rate greater than the first flow rate to deposit the second
phase-changeable material layer on the first phase-changeable material
layer. In some embodiments, the second flow rate may be about ten times
greater than the first flow rate.

[0015]According to still other embodiments of the present invention, a
phase-changeable memory device may include a first chalcogenide compound
configured to switch between a first resistance state and a second
resistance state and doped with nitrogen at a first concentration, and a
second chalcogenide compound formed on the first chalcogenide compound
and doped with nitrogen at a second concentration. The second
concentration can be adjusted to control the diffusion of a material to
the first chalcogenide compound. The second concentration may be higher
than the first concentration.

[0016]In some embodiments, the first chalcogenide compound may be doped
with 0-5 at % nitrogen and the second chalcogenide compound may be doped
with 5-20 at % nitrogen.

[0017]In other embodiments, the phase-changeable memory device may further
include an adhesive layer on the second chalcogenide compound, a first
electrode on the first chalcogenide compound, and a second electrode on
the adhesive layer. The second chalcogenide compound may be between the
adhesive layer and the first chalcogenide compound, the adhesive layer
may be between the second chalcogenide compound and the second electrode,
and the first chalcogenide compound may be between the first electrode
and the second chalcogenide compound.

[0018]In some embodiments, a grain size of the first chalcogenide compound
may be larger than a grain size of the second chalcogenide compound.

[0019]According to still further embodiments of the present invention, the
phase-changeable memory device may include a first electrode and a second
electrode, a phase-changeable memory element between the first electrode
and the second electrode, and an adhesion layer between the
phase-changeable memory element and the second electrode. The
phase-changeable memory element may include a first phase-changeable
layer and a second phase-changeable layer. The first phase-changeable
layer can be switched between a first resistance state and a second
resistance state in response to an electrical signal provided by the
first and second electrodes. The second phase-changeable layer may be
doped with a greater amount of nitrogen than the first phase-changeable
layer. The second phase-changeable layer may not be subjected to phase
change in response to the electrical signal provided by the first and
second electrodes.

[0020]In some embodiments, the concentration of nitrogen can be adjusted
such that the first phase-changeable layer may be subject to a
phase-change but the second phase-changeable layer may not be subject to
a phase change.

[0021]According to yet other embodiments of the present invention, a
method for forming a phase-changeable memory device may include forming a
first phase-changeable layer doped with nitrogen at a first
concentration, and forming a second phase-changeable layer doped with
nitrogen at a second concentration higher than the first concentration.

[0022]In some embodiments, the first phase-changeable layer and the second
phase-changeable layer may be formed by a sputtering process where a
chalcogenide compound is used as a target, an argon gas is used as a
sputtering gas, and a nitrogen gas is used as a nitrogen source.

[0023]In other embodiments, the method may further include forming a first
electrode contacting the first phase-changeable layer, forming an
adhesive layer contacting the second phase-changeable layer, and forming
a second electrode contacting the adhesive layer.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024]FIG. 1 is a cross-sectional view of a phase-changeable memory device
according to some embodiments of the present invention;

[0025]FIG. 2 is a graph illustrating the resistivity of a GST layer
depending on nitrogen doping concentration according to some embodiments
of the present invention;

[0026]FIG. 3 is a graph illustrating the reset current and the set
resistance of a phase-changeable layer depending on the thickness of a
second phase-changeable layer in the phase-changeable memory device of
FIG. 1;

[0027]FIG. 4 is a block diagram of an apparatus for forming a
phase-changeable layer according to some embodiments of the present
invention;

[0028]FIG. 5 is a block diagram of a data processing system including a
memory unit using a phase-changeable memory device according to some
embodiments of the present invention; and

[0029]FIG. 6 is a graph illustrating the titanium concentration profile of
a phase-changeable memory device according to some embodiments of the
present invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0030]The present invention is described more fully hereinafter with
reference to the accompanying drawings, in which embodiments of the
invention are shown. This invention may, however, be embodied in many
different forms and should not be construed as limited to the embodiments
set forth herein. Rather, these embodiments are provided so that this
disclosure will be thorough and complete, and will fully convey the scope
of the invention to those skilled in the art. In the drawings, the size
and relative sizes of layers and regions may be exaggerated for clarity.
Like numbers refer to like elements throughout.

[0031]The terminology used herein is for the purpose of describing
particular embodiments only and is not intended to be limiting of the
invention. As used herein, the singular forms "a", "an" and "the" are
intended to include the plural forms as well, unless the context clearly
indicates otherwise. It will be further understood that the terms
"comprises" and/or "comprising," when used in this specification, specify
the presence of stated features, integers, steps, operations, elements,
and/or components, but do not preclude the presence or addition of one or
more other features, integers, steps, operations, elements, components,
and/or groups thereof. As used herein, the term "and/or" includes any and
all combinations of one or more of the associated listed items.

[0032]It will be understood that, although the terms first, second, third
etc. may be used herein to describe various elements, components,
regions, layers and/or sections, these elements, components, regions,
layers and/or sections should not be limited by these terms. These terms
are only used to distinguish one element, component, region, layer or
section from another region, layer or section. Thus, a first element,
component, region, layer or section discussed below could be termed a
second element, component, region, layer or section without departing
from the teachings of the present invention.

[0033]It will also be understood that when an element or layer is referred
to as being "on", "connected to", "coupled to", or "adjacent to" another
element or layer, it can be directly on, connected, coupled, or adjacent
to the other element or layer, or intervening elements or layers may be
present. In contrast, when an element is referred to as being "directly
on," "directly connected to", "directly coupled to", or "immediately
adjacent to" another element or layer, there are no intervening elements
or layers present.

[0034]Spatially relative terms, such as "beneath", "below", "lower",
"under", "above", "upper" and the like, may be used herein for ease of
description to describe one element or feature's relationship to another
element(s) or feature(s) as illustrated in the figures. It will be
understood that the spatially relative terms are intended to encompass
different orientations of the device in use or operation in addition to
the orientation depicted in the figures. For example, if the device in
the figures is turned over, elements described as "below" or "beneath" or
"under" other elements or features would then be oriented "above" the
other elements or features. Thus, the exemplary terms "below" and "under"
can encompass both an orientation of above and below. The device may be
otherwise oriented (rotated 90 degrees or at other orientations) and the
spatially relative descriptors used herein interpreted accordingly. In
addition, it will also be understood that when a layer is referred to as
being "between" two layers, it can be the only layer between the two
layers, or one or more intervening layers may also be present.

[0035]Embodiments of the invention are described herein with reference to
cross-section illustrations that are schematic illustrations of idealized
embodiments (and intermediate structures) of the invention. As such,
variations from the shapes of the illustrations as a result, for example,
of manufacturing techniques and/or tolerances, are to be expected. Thus,
embodiments of the invention should not be construed as limited to the
particular shapes of regions illustrated herein but are to include
deviations in shapes that result, for example, from manufacturing. For
example, an implanted region illustrated as a rectangle will, typically,
have rounded or curved features and/or a gradient of implant
concentration at its edges rather than a binary change from implanted to
non-implanted region. Likewise, a buried region formed by implantation
may result in some implantation in the region between the buried region
and the surface through which the implantation takes place. Thus, the
regions illustrated in the figures are schematic in nature and their
shapes are not intended to illustrate the actual shape of a region of a
device and are not intended to limit the scope of the invention.

[0036]Unless otherwise defined, all terms (including technical and
scientific terms) used herein have the same meaning as commonly
understood by one of ordinary skill in the art to which this invention
belongs. It will be further understood that terms, such as those defined
in commonly used dictionaries, should be interpreted as having a meaning
that is consistent with their meaning in the context of the relevant art
and/or the present specification and will not be interpreted in an
idealized or overly formal sense unless expressly so defined herein.

[0037]FIG. 1 is a cross-sectional view of a phase-changeable memory device
according to some embodiments of the present invention. Referring to FIG.
1, a phase-changeable memory device 1 includes a phase-changeable layer
21 that is interposed between a first electrode 11 and a second electrode
41. The phase-changeable layer 21 includes a first phase-changeable layer
23 and a second phase-changeable layer 25, and serves as a memory
element. The first phase-changeable layer 23 is different from the second
phase-changeable layer 25 in chemical, electrical, and/or physical
characteristics. For example, the first phase-changeable layer 23 and the
second phase-changeable layer 25 may be different in grain size,
resistivity, chemical composition, nitrogen (N) concentration, and/or
thermal conductivity (Tc). For instance, the second phase-changeable
layer 25 may include a higher concentration of nitrogen than the first
phase-changeable layer 23. In some embodiments, the second
phase-changeable layer 25 may be doped with nitrogen, while the first
phase-changeable layer 23 may not be doped with nitrogen. In other
embodiments, the second phase-changeable layer 25 may contain conductive
materials such as titanium (Ti), zirconium (Zr), tungsten (W), molybdenum
(Mo), tantalum (Ta), copper (Cu), aluminum (Al), titanium tungsten (TiW),
and/or combinations thereof, while the first phase-changeable layer 23
may not contain such elements. In still other embodiments, the second
phase-changeable layer 25 may contain more Ti and/or Zr than the first
phase-changeable layer 23. In still other embodiments, the second
phase-changeable layer 205 may have a larger grain size than the first
phase-changeable layer 23.

[0041]In some embodiments, the phase-changeable layer 21 may be formed of
a chalcogenide compound. Examples of the chalcogenide compound may
include Ge--Sb--Te, As--Sb--Te, As--Ge--Sb--Te, Sn--Sb--Te,
In--Sn--Sb--Te, Ag--In--Sb--Te, Group 5A element-Sb--Te. Group 6A
element-Sb--Te, Group 5A element-Sb--Se, and/or Group 6A element-Sb--Se.

[0042]In some embodiments of the present invention, the second
phase-changeable layer 25 may include a larger amount of nitrogen than
the first phase-changeable layer 23. For example, the first
phase-changeable layer 23 may include about 0-5 atomic percent (at %) of
nitrogen, while the second phase-changeable layer 25 may include about
5-10 at % of nitrogen. As can be seen from FIG. 2, the resistivity of the
phase-changeable layer increases as the nitrogen concentration increases.
In FIG. 2, a germanium-antimony-tellurium (GST) layer is used as the
phase-changeable layer, the x-axis represents the at % of nitrogen
contained in the GST layer, and the y-axis represents the resistivity in
ohm-centimeters (Ωcm) of the GST layer.

[0043]The amount of nitrogen included in the phase-changeable layer
affects the crystalline structure of the phase-changeable layer. A
hexagonal close-packed lattice (HCP) may be dominant when the
phase-changeable layer does not include nitrogen, while a face-centered
cubic arrangement (FCC) may be dominant when the phase-changeable layer
includes nitrogen. In other words, as the concentration of nitrogen
increases, the structure of the crystalline state of the phase-changeable
layer changes from HCP to FCC. The free energy change
(ΔGdoped) between the crystalline and amorphous states of the
phase-changeable layer including nitrogen is smaller than the free energy
change (ΔGundoped) between the crystalline and amorphous
states of the phase-changeable layer that does not include nitrogen.
Accordingly, when the phase-changeable layer includes nitrogen, the
program current (particularly, the reset current) of the phase-changeable
memory device decreases.

[0044]In addition, the phase-changeable layer including nitrogen has a
much smaller grain size than the phase-changeable layer that does not
include nitrogen. Accordingly, when formed with nitrogen as in some
embodiments of the present invention, the phase-changeable layer
increases in resistance, which may reduce the reset current of the
phase-changeable memory device 1.

[0045]In some embodiments of the present invention, when energy such as
heat, light, voltage and/or current is applied to the phase-changeable
layer 21 through the first electrode 11 and the second electrode 41, a
phase change between a set state and a reset state occurs at a portion of
the first phase-changeable layer 23 contacting the first electrode 11.
However, the phase change may not occur at the second phase-changeable
layer 25. The concentration of nitrogen can be adjusted such that the
phase change occurs at the first phase-changeable layer 23 but does not
occur at the second phase-changeable layer 25. As shown in FIG. 1, the
first electrode 11 may have the shape of a contact plug, and thus the
applied current may be concentrated on a portion of the first
phase-changeable layer 23 contacting the first electrode 11. Accordingly,
at least the portion of the first phase-changeable layer 23 contacting
the first electrode 11 increases in temperature and thus undergoes a
phase change.

[0046]Because a phase chance occurs at the first phase-changeable layer
23, the second phase-changeable layer 25 may have different chemical,
electrical and/or physical characteristics from the first
phase-changeable layer 23. The Ti and/or Zr of the adhesive layer 31 may
diffuse and/or otherwise infiltrate into the first phase-changeable layer
23, which may increase a leakage current and/or the reset current of the
phase-changeable memory device. In some embodiments of the present
invention, the second phase-changeable layer 25 may reduce and/or prevent
Ti and/or Zr of the adhesive layer 31 from diffusing and/or otherwise
infiltrating into the first phase-changeable layer 23 where the
phase-change occurs. As described above, the second phase-changeable
layer 25 is higher than the first phase-changeable layer 23 in terms of
nitrogen concentration. However, as the nitrogen concentration increases,
the grain size of the phase-changeable layer decreases. Therefore, the
first phase-changeable layer 23 has a larger grain size than the second
phase-changeable layer 25. Accordingly, the second phase-changeable layer
25 may have a smaller grain size, and thus may function as a protective
layer that is configured to restrain the material (e.g., Ti, Zr, W, Mo,
Ta, Cu, Al, or TiW) of the adhesive layer 31 from diffusing into the
first phase-changeable layer 23. For example, some and/or most of the
material diffusing from the adhesive layer 31 may reach the second
phase-changeable layer 25, but may not reach the first phase-changeable
layer 23.

[0047]Also, the second phase-changeable layer 25 may be formed to reduce
and/or prevent heat applied to the first phase-changeable layer 23 from
being transmitted. That is, the second phase-changeable layer 25 may be
formed to have a relatively low thermal conductivity (Tc). In some
embodiments, the second phase-changeable layer 25 may be formed to have a
lower Tc than the first phase-changeable layer 23. Accordingly, the
second phase-changeable layer 25 may be formed to have an excellent
thermal insulation effect, which may thereby enhance the program
efficiency for the first phase-changeable layer 23.

[0048]In addition, some of the conductive elements included in the
adhesive layer 31 may diffuse into the second phase-changeable layer 25.
Therefore, the second phase-changeable layer 25 may include conductive
elements such as Ti, Zr, W, Mo, Ta, Cu, Al, or TiW. Accordingly, the
second phase-changeable layer 25 may have a reduced resistance, and thus
can be used to form an ohmic contact with the first phase-changeable
layer 23.

[0049]According to some embodiments of the present invention, the second
phase-changeable layer 25 reduces and/or prevents the elements of the
adhesive layer 31 from diffusing into the first phase-changeable layer
23. Accordingly, the adhesive layer 31 can be formed relatively thick,
thereby providing a relatively strong adhesive force between the
phase-changeable layer 21 and the second electrode 41.

[0050]FIG. 3 is a graph illustrating the relationship between the
thickness of the second phase-changeable layer 25 and the reset
current/set resistance of the phase-changeable layer 21, according to
some embodiments of the present invention. As shown in FIG. 3, the
thickness of the second phase-changeable layer 25 is changed while the
overall thickness of the phase-changeable layer 21 is maintained at about
1000 Å. In addition, the first phase-changeable layer 23 is doped to
a nitrogen concentration of about 0.75 at %, while the second
phase-changeable layer 25 is doped to a nitrogen concentration of about
6.5 at %. In FIG. 3, the x-axis represents the thickness in Angstroms
(Å) of the second phase-changeable layer 25, the left side y-axis
represents the reset current Ireset in milliamps (mA), and the right side
y-axis represents the set resistance Rset in kilo-ohms (kΩ). As can
be seen from FIG. 3, the reset current decreases as the thickness of the
second phase-changeable layer 25 increases. This may be due to the fact
that the material diffusion blocking capability of the second
phase-changeable layer 25 may be enhanced as the thickness of the second
phase-changeable layer 25 increases.

[0051]However, in some embodiments, instead of adjusting the thickness of
the second phase-changeable layer 25 as shown in FIG. 3, the nitrogen
concentration of the second phase-changeable layer 25 may be adjusted
while the thickness of the second phase-changeable layer 25 is
maintained. Accordingly, as the nitrogen concentration of the second
phase-changeable layer 25 is increased, the material diffusion blocking
capability of the second phase-changeable layer 25 may be enhanced to
reduce the reset current.

[0052]The nitrogen concentration and/or the thickness of the second
phase-changeable layer 25 may influence the thickness of the adhesive
layer 31 in devices according to some embodiments of the present
invention. As described above, the material diffusion blocking capability
of the second phase-changeable layer 25 may be enhanced as the nitrogen
concentration and/or the thickness of the second phase-changeable layer
25 increases. Therefore, as the nitrogen concentration and/or thickness
of the second phase-changeable layer 25 is increased, a thicker adhesive
layer 31 may be used, which may increase the adhesive force between the
phase-changeable layer 21 and the second electrode 41. If the adhesive
layer 31 is relatively thin, the second electrode 41 may be lifted from
the phase-changeable layer 21. This lifting phenomenon may occur more
frequently in devices having higher degrees of integration. Accordingly,
the phase-changeable memory device 1 of embodiments of the present
invention may be suitable for achieving a relatively high degree of
integration.

[0053]The phase-changeable memory device 1 is programmed by application of
an electrical signal, for example, a program current, to the first
phase-changeable layer 23 through the first and second electrodes 11 and
41. When a relatively high program current is applied to the first
phase-changeable layer 23 for a relatively short time and then the first
phase-changeable layer 23 is cooled rapidly, at least the portion of the
first phase-changeable layer 23 contacting the first electrode 11 becomes
an amorphous state, and thus the phase-changeable memory device 1 is
changed to a reset state with a relatively high resistance. In contrast,
when a relatively low program current is applied to the first
phase-changeable layer 23 for a relatively long time and then the first
phase-changeable layer 23 is cooled, at least the portion of the first
phase-changeable layer 23 contacting the first electrode 11 becomes a
crystalline state, and thus the phase-changeable memory device 1 is
changed a set state with a relatively low resistance.

[0054]Data stored in the phase-changeable memory device 1 may be read by
measuring the resistance of the phase-changeable layer 21. For example,
an electrical signal (e.g., a read current) may be applied through the
first and second electrodes 11 and 41 to the phase-changeable layer 21
(particularly, to the first phase-changeable layer 23), and the resulting
voltage (read voltage) across the first phase-changeable layer 23 may be
compared with a reference voltage by a comparator, such as a sense
amplifier. The read voltage is proportional to the resistance of the
first phase-changeable layer 23. That is, a high read voltage and a low
read voltage indicate a high-resistance state (i.e., a reset state) and a
low-resistance state (i.e., a set state), respectively.

[0055]A method for forming the phase-changeable layer 21 according to some
embodiments of the present invention will now be described in greater
detail with reference to FIG. 4. FIG. 4 is a block diagram illustrating a
deposition chamber used in forming the phase-changeable layer 21
according to some embodiments of the present invention. The first
phase-changeable layer 23 and the second phase-changeable layer 25 may be
deposited in situ using the same deposition chamber, or may be deposited
using different deposition chambers.

[0056]Referring now to FIG. 4, an apparatus for depositing a chalcogenide
compound according to some embodiments of the present invention includes
a reaction chamber 301 having a substrate 305 and a chalcogenide compound
target 307 that face each other. A direct current (DC) pulse generator is
connected between the chalcogenide compound target 307 and the substrate
305 to supply a DC pulse to the chalcogenide compound target 307 and the
substrate 305. The substrate 305 is supported by a support 303. A magnet
309 is mounted on a surface of the chalcogenide compound target 307
opposite the substrate 305. Therefore, a higher-density plasma may be
formed at a portion of the chalcogenide compound target 307 corresponding
to the magnet 309 than at the other portions in the reaction chamber 301.
Accordingly, more target elements may be emitted to increase the speed of
deposition of a thin film on the substrate 305. The chalcogenide compound
target 307 may be formed of Ge--Sb--Te, As--Sb--Te, As--Ge--Sb--Te,
Sn--Sb--Te, In--Sn--Sb--Te, Ag--In--Sb--Te, Group 5A element-Sb--Te,
Group 6A element-Sb--Te, Group 5A element-Sb--Se, and Group 6A
element-Sb--Se.

[0057]The wall of the reaction chamber 301 is equipped with a gas supply
duct 313 through which an inert gas and a nitrogen gas flow into the
reaction chamber 301. In addition, a exhaust duct 315 for exhausting the
reaction byproducts in the reaction chamber 301 is connected to the
reaction chamber 301. The reaction chamber 301 is maintained at a
high-vacuum state by a vacuum pump.

[0058]The inflow rate of nitrogen gas supplied through the gas supply duct
313 may be used to control the nitrogen concentration of the first and/or
second phase-changeable layers. For example, nitrogen gas may be supplied
at a flow rate of about 2 standard centimeter cube per minute (sccm) to
form the first phase-changeable layer 23, and nitrogen gas may be
supplied at a flow rate of about 25 sccm to form the second
phase-changeable layer 25. The inside of the reaction chamber 301 may be
maintained at a pressure range of about 0.1-1 mT and at a temperature of
about 100-350° C.

[0059]The DC pulse generator 311 may supply the chalcogenide compound
target 307 and the substrate 305 with a positive DC pulse, a negative DC
pulse, and/or a DC pulse that swings between a positive value and a
negative value. The DC pulse generator 311 includes a DC bias supply
source 311a for generating a DC bias voltage and a pulse converter 311b
for converting the generated DC bias voltage into a square-wave pulse
voltage. Methods for generating a pulse voltage using a DC bias voltage
are well known to those skilled in the art, and thus their description
will be omitted for conciseness.

[0060]A nitrogen gas and an inert gas such as an argon gas are supplied
through the gas supply duct 313 into the reaction chamber 301 at a
predetermined flow rate. The argon and nitrogen gases in the reaction
chamber 301 are changed into a plasma state by high-voltage pulses that
are supplied from the DC pulse generator 311 to the chalcogenide compound
target 307 and the substrate 305. Argon ions (Ar.sup.+) in a plasma state
collide against the surface of the chalcogenide compound target 307 at a
high energy, and thus the elements of the chalcogenide compound target
307 are separated from the surface of the chalcogenide compound target
307. The separated elements and nitrogen radicals react with each other,
and thus a chalcogenide compound thin film including nitrogen is
deposited onto the substrate 305.

[0061]In some embodiments, the amount of nitrogen gas flowing into the gas
supply duct 313 may be controlled such that the first phase-changeable
layer 23 and the second phase-changeable layer 25 can be formed in situ
using the same deposition apparatus. For example, nitrogen gas may be
supplied at a first flow rate to form the first phase-changeable layer 23
with a desired thickness, and nitrogen gas may be supplied at a second
flow rate higher than the first flow rate to form the second
phase-changeable layer 25 with a desired thickness.

[0063]Referring back to FIG. 1, the first electrode 11 is formed in the
shape of a plug that may extend through an insulating layer. The forming
of the first electrode 11 may include depositing an insulating layer,
forming a contact hole in the deposited insulating layer, depositing a
conductive layer for the first electrode 11, and etching the portions of
the deposited conductive layer outside of the contact hole. The
conductive layer for the first electrode 11 may be deposited by physical
vapor deposition (PVD), CVD, sputtering, and/or ALD.

[0064]The second electrode 41 may be formed by depositing a conductive
layer for the second electrode 41 and then patterning the deposited
conductive layer by using a photolithography process. The conductive
layer for the second electrode 41 may be deposited by PVD, CVD,
sputtering, and/or ALD. Likewise, the adhesive layer 31 may be formed by
a deposition process and a patterning process using a photolithography
process.

[0065]In some embodiments, the phase-changeable layer 21, the adhesive
layer 31, and the second electrode 41 may be formed by a single
patterning process. For example, after thin films for the
phase-changeable layer 21, the adhesive layer 31, and the second
electrode 41 are sequentially formed on an insulating layer on which the
first electrode is formed, a photolithography process may be used to
pattern the thin films until the insulating layer is exposed, thereby
forming the phase-changeable layer 21, the adhesive layer 31, and the
second electrode 41.

[0066]FIG. 5 is a block diagram of a data processing system 50 including a
memory 52 using phase-changeable memory devices according to some
embodiments of the present invention. Referring to FIG. 5, the data
processing system 50 includes a central processing unit (CPU) 54 that
communicates with an input/output (I/O) unit 56 via a bus 58. Examples of
the CPU 54 may include a microprocessor, a digital signal processor,
and/or a programmable digital logic unit. Under the control of a memory
controller, the memory 52 communicates with the system via the bus 58.
For example, when the data processing system 50 is a computer system, it
may include peripheral devices such as a floppy disk drive (FDD) 60 and a
CD-ROM drive 62. These peripheral devices communicate with the CPU 54 via
the bus 58. The memory 52 may include one or more resistance memory
elements. Also, the memory 52 may be combined with the CPU 54 to form a
single integrated circuit (IC).

[0068]Further embodiments of the present invention will now be described
with reference to the graph of FIG. 6. In FIG. 6, a first electrode of
TiAlN, a first GST layer containing about 3-at % nitrogen, a second GST
layer containing about 10-at % nitrogen, a Ti adhesive layer, and a
second electrode of TiN are sequentially deposited on a substrate, and
the element concentrations of the deposited layers (with reference to
depths from a surface of the adhesive layer) are illustrated.

[0069]As shown in FIG. 6, the second GST layer has a greater nitrogen
concentration than the first GST layer, and thus includes smaller amounts
of Ge, Sb and Te than the first GST layer. As can be seen from FIG. 6,
the amount of the Ge, Sb and Te is maintained to be substantially
constant over a depth range of about 500-1000 Å, but decreases as the
depth decreases below about 500 Å. The portion where the amount is
substantially constant corresponds to the first GST layer, and the
portion where the amount decreases corresponds to the second GST layer.

[0070]As can be seen from FIG. 6, the titanium concentration gradually
decreases from the Ti adhesive layer toward the second GST layer, and
rapidly decreases from the second GST layer toward the top of the first
GST layer. Consequently, the first GST layer has very little titanium
concentration. This illustrations that a major portion of the titanium is
absorbed at the second GST layer contacting the Ti adhesive layer and
thus relatively little to no titanium is diffused into the first GST
layer. Therefore, the second GST layer substantially reduces and/or
prevents titanium from the adhesive layer from diffusing into the first
GST layer.

[0071]As described above, some embodiments of the present invention may
make it possible to reduce the program current of a phase-changeable
memory device. Also, it may be possible to implement phase-changeable
memory devices that have relatively high reliability and/or integration
level. Also, it may be possible to enhance the adhesive force between the
phase-changeable layer and the second electrode in phase-changeable
memory devices according to some embodiments of the present invention.

[0072]While the present invention has been particularly shown and
described with reference to preferred embodiments thereof, it will be
understood by those skilled in the art that various changes in form and
details may be made therein without departing from the spirit and scope
of the invention as defined by the appended claims.